1. Field of the Invention
The present invention relates to a technique for reducing the size of a discharge lamp lighting circuit and the cost to manufacture the same, and for equalizing time intervals at which high tension pulses are generated.
2. Description of the Related Art
There is known a lighting circuit for lighting up a discharge lamp (e.g., metal halide lamp), which is made up of a DC power source circuit, a DC-AC converting circuit, and a start-up circuit (called a starter circuit). In its application to a light source of a vehicular lighting device, for example, a DC-DC converting circuit (DC-DC converter) is used for the DC power source circuit. A full bridge circuit (=a circuit consisting of four semiconductor switching elements which are on/off controlled in the form of two pairs of switching elements) and its drive circuit are used for a DC-AC converting circuit. The output voltage of the DC-DC convertor is converted into a voltage of a rectangular waveform in the full bridge circuit, and the converted one is applied to the discharge lamp.
To start up the discharge lamp, to generate high voltage necessary for breaking down the discharge lamp, a switching element, e.g., a spark gap element, and a capacitor for storing charge are provided in the primary side circuit including the primary winding of a transformer (starter transformer). Energy produced when the switching element is rendered conductive and the capacitor is discharged generates a start-up pulse, and the pulse is supplied to the discharge lamp.
The following circuits may be enumerated for the circuit for charging the capacitor.
1) A dedicated high tension winding is connected to a transformer of the DC-DC converting circuit, and an output voltage derived from the winding is used for charging the capacitor.
2) A switching power source is used for the DC-DC converting circuit. A supply voltage necessary for charging the capacitor is produced by utilizing the switching operation based on its output control.
3) A supply voltage necessary for charging the capacitor is produced by utilizing the switching operation based on the output control of the DC-AC converting circuit.
In the charging circuit of 1) above, a dedicated winding is provided at the secondary winding of the transformer. The output voltage of the winding is rectified and smoothed, and the resultant voltage is used. In the circuit 2) or 3) above, a charge transfer circuit, a charge pump circuit or the like, which is made up of a plurality of capacitors and resistors, is disposed in the output stage of the DC-DC converting circuit or the DC-AC converting circuit. A voltage necessary for charging the capacitor of the start-up circuit is produced by such a circuit arrangement.
However, the related charging circuits have the following problems.
In the case of the above 1), an electrical load is applied to the transformer of the DC-DC converting circuit. Additional use of the high tension winding brings about size increase of the transformer. A necessity of using a breakdown withstanding structure arises in order to prevent the corona discharge from occurring. When the covered cord of the multi-layered type is used for the high tension winding, instead of employing the breakdown withstanding structure, cost-increase problem arises anew.
As to the circuit of the above 2), it suffers from the following problems. The number of electronic parts used is increased. High breakdown withstanding parts must be used. The conduction intervals at which the switching element of the start-up circuit is made conductive are instable.
FIG. 17 is presented for explaining the problems of the related art. An upper graph in the figure roughly shows a variation of the output voltage xe2x80x9cVdcxe2x80x9d of the DC-DC converting circuit and a switching frequency SH (=a frequency at which the switching element of the DC-DC converting circuit is turned on and off) with respective to time. In either case, a start point of time xe2x80x9ctxe2x80x9d is a lighting start time point (=time point of turning on the lighting switch).
As shown, the output voltage xe2x80x9cVdcxe2x80x9d swiftly rises, and reaches a voltage (xe2x80x9cO.C.V.xe2x80x9d=open circuit voltage) and depicts a flat curve. The switching frequency SH is continuously high till the output voltage xe2x80x9cVdcxe2x80x9d reaches the O.C.V. (viz., the switching element is frequently turned on and off). Subsequently, the load is light and hence the switching frequency is low, viz., the switching element is intermittently turned on and off. Where the switching frequency (=number of switching times) is low, the amount of charge supplied to the capacitor in the start-up circuit is lessened. As a result, a time taken for the switching element (spark gap element or the like) in the start-up circuit to be conductive is long.
A lower graph in FIG. 17 shows its sate, and a variation of the voltage xe2x80x9cVCxe2x80x9d across the capacitor in the start-up circuit with respective to time. In the figure, a time point where the voltage xe2x80x9cVCxe2x80x9d rises and rapidly decreases indicates a time point where the switching element starts its conduction. A start point of time xe2x80x9ctxe2x80x9d is a lighting start time point.
As seen from the figure, when the switching frequency SH is high, the voltage xe2x80x9cVCxe2x80x9d rapidly rises, while when the switching frequency SH is low, the voltage xe2x80x9cVCxe2x80x9d gently rises. Accordingly, with regard to the time intervals at which the high tension pulses (or the start-up pulse) are generated by the start-up circuit, a first start-up pulse is short in width and quickly generated. A second start-up pulse and the subsequent ones are wide and generated at slow generation timings.
The circuit of 3) above needs a capacitor of large capacity, and hence has the problem of cost and size reduction. This is due to the fact that the switching frequency in the DC-AC converting circuit is much lower than that in DC-DC converting circuit. In this sense, the problem may be solved by increasing the switching frequency. Where it is high, the problem of the acoustic resonance phenomenon of the discharge lamp arises anew, however. Therefore, there is a limit in increasing the switching frequency (generally, several hundreds Hz).
Thus, the related techniques are incapable of satisfactorily meeting the requirements of the circuit size and cost reduction. The techniques have still some problems in generating the start-up pulse at fixed time intervals.
Accordingly, an object of the present invention is to provide a discharge lamp lighting circuit which is suitable for the circuit size and cost by simplifying a circuit for producing a supply voltage necessary for generating a start-up pulse to be applied to a discharge lamp, and to generate start-up pulses at fixed time intervals.
To achieve the above object, there is provided a discharge lamp lighting circuit having a DC-DC converting circuit, a DC-AC converting circuit provided at the post stage of the DC-DC converting circuit, and a start-up circuit for generating a start-up pulse for application to a discharge lamp. The discharge lamp lighting circuit is characteristically constructed as follows.
In the discharge lamp lighting circuit, the start-up circuit includes a transformer, a primary side circuit having the primary winding of the transformer includes a first capacitor and a switch element, when the switch element is conductive, charge stored in the first capacitor is discharged, and a high voltage pulse generated in the secondary winding of the transformer is supplied as a start-up pulse to the discharge lamp.
The DC-DC converting circuit includes a transformer and a switch element for the output control, a first rectifying element and a smoothing capacitor are provided at the output stage of the transformer, and energy stored in the transformer is stored into the smoothing capacitor through the first rectifying element by an on/off control of the switching element.
The DC-AC converting circuit includes a bridge circuit using a plurality of switching elements, and the output signal of the DC-DC converting circuit is output, in the form of an alternate current varying at a fixed or variable frequency, through alternating operations of the switching elements by the on/off control.
A circuit includes a second capacitor and a second rectifying element between one of the output terminals of the DC-AC converting circuit and a ground line or a line equivalent to it, and the second capacitor is charged with current flowing in the direction as defined by the second rectifying element connected to the line.
A circuit includes a third capacitor and a third rectifying element is disposed between a node between the secondary winding of the transformer in B) above and the first rectifying element and a node on a line connecting the second capacitor and the second rectifying element, and the third capacitor is charged with current flowing in the direction as defined by the third rectifying element connected to a connection line connecting the second capacitor and the second rectifying element.
A node between the third capacitor and the third rectifying element is connected to a voltage supply line to the first capacitor in A) above to charge the capacitor.
According to the present invention, the discharge lamp lighting circuit charges the second and third capacitors by the utilization of the switching controls of the DC-DC converting circuit and DC-AC converting circuit, whereby, the supply voltage necessary for charging the first capacitor in the start-up circuit is obtained. Accordingly, the number of parts used is relatively small and use of a capacitor of small capacitance is allowed. It is noted that the switching control of the DC-AC converting circuit as well as that of the DC-DC converting circuit is utilized. Therefore, the time intervals at which the start-up pulses are generated are substantially equal and hence equalized.